[Ambientnoise]

Mike, I think you need to consider what you want (in terms of both the end result and your effort) in a bit more detail. For example if you want say 25v to 5v, adjustable at 15A max you will need to manage well over 300W of heat if you use pure linear regulators like those above. This is serious heatsink/fan territory. In this case I guess a thyristor pre regulator operating at 50Hz, is the way to go to limit dissipation in the pass devices. The modern approach would be switchers operating at say 100kHz but you are getting into major non trivial design.

Ken